Multi-bed, down-flow catalytic reactors are used in the petroleum and chemical industries for a variety of processes. In the petroleum refining industry, multi-bed, down-flow reactors are used in processes for hydrotreating, hydrodesulfurization, hydrofinishing, hydrocracking, and dewaxing. In these processes, the process liquid is mixed with a gas or vapor and this multiple phase mixture is passed through the packed catalyst beds. In exothermic reactions, large amounts of heat may be generated in the catalyst beds. Quench media may be added to regulate the temperature in the reactor. Typically, the quench media is also a reactant, e.g., hydrogen. As the liquid and gas mixture flows through a catalyst bed, some maldistribution of temperature and reactant concentration often develops across each bed. Unless this is mitigated, the reactor operation would be inefficient and hot spots may develop that could potentially shorten catalyst life and force premature reactor shutdown and therefore plant shutdown. It is, therefore, necessary to provide reactor internals for collecting and mixing the liquid and gas exiting from one bed before distribution to the next bed, so that any such maldistribution is not propagated from one catalyst bed to the next. To accomplish this, distributor assemblies are used between the catalyst beds to ensure that the distribution of the liquid and gas mixture (and as the case may be, the vaporous reactant or quench gas) across each bed is as uniform as possible in both composition and temperature. To make the corrections of temperature and concentration maldistributions, the distributor assembly should provide for good liquid-liquid, gas-gas, and gas-liquid mixing. In addition to providing good mixing, it is desirable that the distributor assembly be compact and impose only a low pressure drop.
By providing such uniform distribution, each bed will be efficiently utilized and the desired catalytic reactions will take place in a more predictable manner. Numerous types of distributor assemblies for multi-bed, down-flow reactors are known in the prior art. One example is shown in U.S. Pat. No. 4,836,989, in which the distributor assembly includes a collector tray arranged below a catalyst bed and a distributor tray is mounted under the collector tray. The distributor tray is fed by spillways which provide passageways for the liquid collected on the collector tray, as well as quench gas injected between the distributor tray and the catalyst bed. The spillways have outlets below the collector tray which face sideways and tangentially into an annular second mixing zone located under the collector tray. This second mixing zone has a high liquid holdup. The outlets are described as imparting a swirling motion to the liquid in the annular second mixing zone that promotes good mixing and temperature equilibrium. Further mixing and redistribution apparatus is provided below the second mixing zone to collect and distribute liquid over the underlying catalyst bed. The design also requires a high pressure drop to be effective.
Another type of a distributor assembly, as shown in U.S. Pat. No. 4,960,571, includes a collector plate that has a central opening therein. A quench mixing zone is provided under the collector plate, in which a second plate having openings formed in an annular outer portion thereof is mounted under the collector plate, with the annular portion thereof being situated radially outwardly of the central opening in the collector plate. Baffles are affixed to the annular portion and arranged at a tangential angles and in concentric rings to induce current eddies in the liquid and gas flowing through the annular portion. The design does not provide as extensive radial mixing as is usually desirable.
Another distributor assembly for multi-bed, down-flow reactors is described in U.S. Pat. No. 5,690,896 ('896 patent). The '896 patent teaches a mixing box arrangement were all fluid collected on a tray flow in a spiral pattern to a single mixing zone leading to a single outlet. This design has drawbacks in that the degree of correction of temperature and composition distributions may depend upon the origin of the distribution relative to the single opening to the mixing zone. Also, because of the single opening and mixing zone, the required height of the assembly could be high in order to accommodate a sufficient liquid throughput. The design also requires a high pressure drop to be effective.
Examples of other types of distributor assemblies for multi-bed, down-flow reactors are described in U.S. Pat. Nos. 3,705,016, 3,977,834, and 4,182,741. It would be advantageous to have a distribution assembly that overcame the deficiencies of known designs. The distributor assembly of the invention provides this improved design.